The present disclosure relates to amplifier circuits, and more particularly to amplifier circuits with output filtering.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Some amplifier circuits require filtering at an output thereof to reduce noise. It may be difficult to provide filtering without adversely impacting a corner frequency of the amplifier circuit. Referring now to FIG. 1, an amplifier circuit 100 includes an amplifier 110 having inverting and non-inverting inputs. The non-inverting input may communicate with a reference potential such as ground.
An input signal VIN is coupled through an input resistance RI to the inverting input.
A bypass capacitance Cb has one end that communicates with the inverting input and an opposite end that communicates with the output of the amplifier 110. A load resistance RL has one end that communicates with the output of the amplifier 110, which has a gain A. A feedback resistance RF has one end that communicates with the inverting input of the amplifier 110 and an opposite end that communicates with an opposite end of the load resistance RL. For example only, the input resistance RI and feedback resistance RF may be substantially equal resistance values, such as a resistance R. In the description that follows, R=RF=RI.
A load capacitance CL has one end that communicates with the opposite end of the load resistance RL and an opposite end that communicates with the reference potential. Another capacitance CL′ has one end that communicates with the output of the amplifier 110 and an opposite end that communicates with the reference potential. An output voltage VOUT is taken at the one end of the load capacitance CL. The load resistance RL and the load capacitance C2 provide filtering at the output of the amplifier circuit.
Referring now to FIGS. 2 and 3, operating characteristics of the amplifier circuit are shown. In FIG. 2, a low-frequency or DC equivalent circuit of the amplifier circuit 100 is shown. At low-frequency, the capacitances in the circuit of FIG. 1 are open circuits. Noise at the input is amplified and output. In FIG. 3, a high-frequency equivalent circuit of the amplifier circuit 100 is shown. At high-frequency, the capacitances in the circuit of FIG. 1 are short circuits. As can be appreciated, the value of the capacitance Cb must be sufficiently large for the circuit to operate correctly.
Referring now to FIGS. 4 and 5, an open loop response of the circuit of FIG. 1 is shown. In FIG. 5, the gain of the amplifier increases and then levels off at a corner frequency that is approximately equal to
      1                  C        b            ⁢              R        2              .As discussed above, the value of the bypass capacitance Cb should be relatively large, which increases the corner frequency. Some applications may require the corner frequency to occur at a lower frequency while still providing output filtering.